1. Field of the Invention
The present invention relates to a toner concentration sensor, as well as a toner concentration control method, for detecting a toner concentration of a developing unit to be used in image forming apparatuses such as copiers and facsimiles.
2. Description of the Related Art
Developers, or developing powders, for use in developing units come in two types, one-component developer and two-component developer. The two-component developer is fabricated by mixing magnetic carrier particles and nonmagnetic toner particles together. Toner particles mixed in the magnetic carrier particles at a proper mixing ratio adhere to a latent image part on a photoconductor drum, by which a toner image is formed.
Therefore, with the two-component developer used for development, out of the magnetic carrier particles and the nonmagnetic toner particles, the nonmagnetic toner particles alone are consumed while the magnetic carrier particles are circulated and repetitively used in the developing unit.
As a solution, a toner concentration sensor for detecting a toner concentration in the developing unit is provided on the developing unit, so that toner is supplied as required from a toner supply unit to the developing unit based on a toner concentration detection result by this toner concentration sensor. Along with this, the two-component developer, which is a carrier-and-toner mixture, is stirred in the developing unit so that the carrier-and-toner mixing ratio becomes more uniform in the developing unit.
As to this type of toner concentration sensor, some are so designed that changes in magnetic permeability of a two-component developer, which is a carrier-and-toner mixture, are detected by changes in resonance frequency of an LC resonance circuit to detect a toner concentration.
A resonance frequency f of an LC resonance circuit composed of an inductance L and a capacitance C can fundamentally be determined by the following equation (1):f=(2π(L·C)1/2)−1  (1)
Then, the inductance L of the coil and the capacitance C of the capacitor have temperature characteristics, respectively. Because of this, changes in temperature in an environment in which the toner concentration sensor is installed causes the LC resonance circuit to change in oscillation frequency.
As a result, there has been a problem that the toner concentration sensor using the LC resonance circuit is subject to changes in output of the LC resonance circuit due to environment temperatures.
Thus, in a toner concentration sensor disclosed in Literature 1 (JP 2000-347495 A), detection errors of toner concentration due to temperature changes are compensated by using a differential transformer and a temperature compensation capacitor.
Also, in a toner concentration sensor disclosed in Literature 2 (JP H10-062390 A), effects of temperature on the resonance frequency are reduced by the setting that the coil and the capacitor, of which the LC resonance circuit is made up, are given by those having reverse temperature characteristics.
On the other hand, toner concentration sensors are under a desire for further reduction in detection errors of toner concentration.